This invention is directed to the anchoring of offshore platforms and vessels ("vessels" herein) such as, but not limited to floating or semi-submersible types. In particular, the invention is directed to method and apparatus for deploying a plurality of anchors and anchor lines for such offshore vessels. A typical offshore drilling vessel is moored in position by the use of eight or more anchors and associated anchor lines. The anchors are deployed in a spread pattern to resist the environmental forces of wind, wave, and currents tending to move the vessel off of its desired location. The final location of each anchor is by no means insignificant. The location and holding power of the anchor and the length and breaking strength of the anchor line must be designed to take into effect the potential environmental fores to be encountered. A drilling rig is connected to the sea bottom by a riser tube, hence a vessel is limited to the horizontal distance it may move off the well location to avoid bending the riser tube, and/or exceeding the breaking strength of the anchor line.
In substantially deep water, the deployment of an anchor from an offshore vessel requires a workboat of extensive horsepower plus other power sources and equipment such as a winch or windlass which take and retain the anchor on board. The anchor end may be connected to the winch by a wire pendant line. The remaining anchor line consists of a chain, wire line or combination of both connected at the working end to the anchor and at its `bitter end` to the vessel via an anchor windlass. The purpose of the workboat is to take the anchor to a predetermined location from the offshore vessel which is typically marked by an anchoring buoy. As the workboat proceeds toward the location, the anchor line is run out or payed out by the windlass located on the offshore vessel. The payout of the anchor line is controlled by some form of a brake or retarding device also operated by the windlass operator. Typical of such brakes are water brakes registered as are manufactured by Parmac, Inc. under the trademark "Hydromatic". When the workboat reaches the desired anchor buoy or anchor location, additional payout of the anchor line is stopped. While maintaining the anchor position, the workboat lowers the anchor to the bottom by the use of its winch and the anchor pendant line. The anchor marking buoy is then attached to the end of the pendant line by the workboat crew. Once the anchor has reached bottom, the windlass operator hauls in or pulls in on the anchor line with the windlass to set the anchor and place the desired amount of tension and scope to the line.
A typical windlass is mounted on two pivot points on the offshore vessel with a load measuring device installed between a third mounting point and the windlass. The load measuring device consists of a hydraulic or electronic device capable of sensing the tension applied to the anchor line by the windlass. A load or tension indicator is connected to the load measuring device and is located in the windlass operator's cab. One such complete windlass operating system, including the cab is located at each corner of the offshore vessel. Also installed in connection with the windlass is a mechanical or electronic length measuring device with a digital indicator located in the windlass operator's control cab. Additional instrumentation located in the operator's cab includes an indicator of the payout speed of the anchor line and the anchor line length still on board the vessel.
The deployment of the anchor and its associated chain and/or wire line involves several problems, the most important of which is to maintain the anchor line from dragging the bottom. The drag of the anchor line or friction on the bottom surface increases the bollard pull to a point where the workboat can become stalled in the water. In addition, it is not at all uncommon to find submerged objects, such as pipelines, well equipment, treating equipment at the bottom of the sea, that is to be avoided by the anchor line due to the damage or entanglement that may result. As such, the windlass operator has no way to determine where the anchor line is located in the water since he cannot visually see the line or the bottom. The tension indicator, previously described, will change abruptly whenever the anchor line touches bottom, but only afterward and not before. The workboat Captain must rely primarily on the workboat speed indicator and throttle sections as an indication of the bollard pull. In addition, his own personal `feel.degree. of the boat provides indications to him of the vertical load and the bollard pull relative to the tension created from the anchor line.
Prior to setting the anchor, the water depth at the anchor and at the rig site is usually known, and maps are provided to show the contour of the sea bed between the well site and the anchor. Despite the fact that the windlass operator has knowledge of the bearings of the anchor as to its distance and location and depth of water, he still is unable to determine the actual profile of the anchor line is positioned in the water.